The present invention relates to bulk acoustic wave devices such as acoustic resonators, and more particularly to acoustic mirror materials used in these devices.
In recent years, much research has been performed in the development of bulk acoustic wave devices, primarily for use in cellular, wireless and fiber-optic communications, as well as in computer or computer-related information-exchange or information-sharing systems. There is a trend in such systems for operation at increasingly higher carrier frequencies, principally because the spectrum at lower frequencies has become relatively congested, and also because the permissible bandwidth is greater at higher frequencies. Piezoelectric crystals have provided the basis for bulk acoustic wave devices for filtering or frequency control such as oscillators, acoustic resonators and filters, operating at very high radio frequencies (on the order of several gigahertz).
In many high-frequency applications, filters such as band pass and/or band stop filters are based on dielectric-filled electromagnetic cavity resonators with physical dimensions that are large, since they are dictated by the wavelength of the resonating electromagnetic wave. Due to the interaction between electrical charge, stress, and strain in a piezoelectric material, such material acts as a transducer, which converts energy back and forth between electromagnetic and acoustic (i.e., mechanical) energy. Thus, a piezoelectric material incorporated in a structure designed to have a strong mechanical resonance provides an electrically resonant device.
The velocity of an acoustic wave, however, is approximately {fraction (1/10000)} that of the velocity of an electromagnetic wave. This relationship between the wave""s velocity and device dimensions thus allows a reduction of roughly this factor in the size of certain devices, including acoustic resonators, employing this material. In other words, an electrical filter based on acoustic waves may be much smaller than one based on electromagnetic waves. To achieve this size reduction, one must use a mechanical material that transduces between electromagnetic and mechanical energies. For example, such may be a piezoelectric, magnetostrictive or electrostrictive material. Although the discussion below focuses on piezoelectric materials for transduction, the transduction is not limited to these materials, but may by attained using one of the other above-noted materials, and/or a combination thereof.
Acoustic resonator devices containing piezoelectric materials, such as thin film resonators (hereinafter xe2x80x9cTFRxe2x80x9d) are typically used in high-frequency environments ranging from several hundred megahertz (MHz) to several gigahertz (GHz). FIG. 1 illustrates a side view of a conventional TFR component 100. In FIG. 1, TFR component 100 includes a piezoelectric material 110 interposed between two conductive electrode layers 105 and 115, with electrode layer 115 formed on a support structure 120.
The support structure 120 can be a plurality of alternating reflecting layers (acoustic mirrors) provided on a solid semiconductor substrate which may be made of silicon, sapphire, glass or quartz, for example. The support structure 120 can alternatively be removed after device fabrication leaving a free standing membrane type device. The piezoelectric material is commonly selected from the group comprising at least ZnO, CdS and AlN. Electrode layers 105 and 115 are generally formed from a conductive material, such of Al, but may be formed from other conductors as well.
These acoustic resonator devices are often used in filters, more particularly in TFR band pass and/or band stop filter circuits applicable to a myriad of communication technologies. For example, TFR filter circuits may be employed in cellular, wireless and fiber-optic communications, as well as in computer or computer-related information-exchange or information-sharing systems.
In these acoustic devices, any sympathetic vibration or mode has a response curve showing how at a certain frequency the amplitude of mechanical response goes through a peaked maximum for fixed excitation strength. Because of the coupling of mechanical and electrical energy in a piezoelectric film, there is also an electrical current response peak where the film produces a maximum current for a fixed voltage since the mechanical motion produces charge at the surface of the piezo. This peak defines the xe2x80x9czeroxe2x80x9d resonant frequency of a acoustic resonator (i.e., the term refers to the zero or low impedance to the flow of current.)
The piezoelectric film also produces a polarization current like a capacitor, since it is a dielectric. This current increases linearly with frequency for a fixed voltage, and since the mechanical resonance is narrow (about 1 part in 100 changes in frequency) the polarization current can be essentially thought of as constant while the piezoelectric current goes through its peak.
In view of the above, a second electrical feature, the xe2x80x9cpolexe2x80x9d frequency or frequency of highest impedance can now be understood. The pole frequency is the frequency at which the piezoelectric and polarization currents most nearly or substantially cancel each other out. Since the piezoelectric current has a peak and reverses its polarity through resonance, its value essentially cancels that of the nearly constant polarization current at one frequencyxe2x80x94the pole frequency. Therefore, the separation between pole and zero resonant frequencies is dependent in one respect on the properties of the piezoelectric material, and the mechanical resonance that is explained in more detail below.
As discussed above, the separation in frequency between pole and zero resonant frequencies in an acoustic device such as a TFR is determined by properties of the piezoelectric film, and more particularly by the mechanical and electrical properties and/or characteristics of the piezoelectric. For a piezoelectric film comprised of AlN, the fractional separation is about 3% of the resonance frequency for a plate geometry suspended in air, such as in a membrane type TFR (e.g. the lateral dimension is large as compared to the film""s thickness).
FIG. 2 illustrates a conventional Bragg reflector stack constituting acoustic mirror layers of an acoustic device such as a TFR. The reflecting stack 125 of FIG. 2 corresponds somewhat to substrate 120 of FIG. 1, and illustrates the make up of the acoustic mirrors in more detail. Referring to FIG. 2, the conventional acoustic mirror arrangement or Bragg reflecting stack 125 comprises a plurality of alternating mirror layers of a high acoustic impedance material, such as AlN layers 130a-d, and a low acoustic impedance material such as SiO2 layers 135a-d, that are provided on a substrate 140 such as silicon, for example. Other conventional reflecting stacks used in acoustic devices such as TFRs typically include acoustic mirror layer combinations of Si3N4xc2x1x/SiO2 as well.
Acoustic impedance is the product of a material""s density and speed. This relation is important because, as a sound wave passes between two unlike materials, the portion of the wave reflected by the interface therebetween is larger for a greater difference in impedance between the differing materials. Accordingly, a common theme of the inventors is that, in order to fabricate a good acoustic mirror, and hence acoustic resonant device, it is desirable to place materials having as dissimilar impedance as possible against each other for maximal reflections.
However, there is at least one drawback in fabricating acoustic resonator devices such as TFRs with the reflecting stack 125 illustrated in FIG. 2. The mechanical resonance, and hence the piezoelectric current response peak of the resonator is altered when a device is fabricated on an acoustic mirror, as compared to the case where the device is fabricated as a membrane. Unfortunately, for a resonator solidly mounted on an acoustic mirror, the pole frequency is reduced to a value closer to the zero frequency of the resonator. Moreover, use of conventional acoustic mirrors comprised of the above-noted AlN/SiO2 layers (as well as the Si3N4xc2x1x/SiO2 layer combination) disadvantageously reduce this separation to around 2%, down from the 3% achievable when an AlN piezoelectric layer is in a membrane form.
This is an undesirable characteristic, because in filter design the maximum bandpass filter bandwidth achievable is proportional to the pole-zero separation. Additionally, important filter applications such as GSM (global system for mobile communications, a cell phone standard widely used in non-US based systems) require substantially all or as much of the bandwidth that an AlN membrane-based TFR filter can provide. Thus, using a non-optimal acoustic mirror may prohibit AlN from being used as the piezoelectric material in some of the larger market filter applications, such as GSM noted above.
Further, the current acoustic mirror arrangement requires a substantial number of alternating reflective layers, providing for a bulkier resonator device that requires longer manufacturing process time and cost. This is because the number of layers required depends on the degree of acoustic mismatch between them, and the AlN/SiO2 and Si3N4xc2x1x/SiO2 layer combinations currently used do not represent the highest possible impedance mismatch. To restate, at each interface there is a reflection of energy; and if each reflection is weak because of poor impedance mismatch, more layers are therefore required to achieve the total desired reflection.
Accordingly, the greater the acoustic impedance mismatch between the piezoelectric layer and the topmost or first acoustic mirror layer, the closer the acoustic device approaches its maximum achievable bandwidth, or limit achievable, as compared to the case where the layer was made of air, for example. Air presents an enormous impedance mismatch with any solid material, simply because its density is typically a factor of at least 1000 lower than the density of the adjacent solid material. Maximal impedance mismatch is also important, albeit to a lesser extent, for the interfaces between each pair of subsequent mirror layers, since it is always beneficial to have a large acoustic impedance mismatch, when maximum bandwidth and fewer required layers are desired. Therefore, what is needed is a reflecting stack for an acoustic device which can attain the highest possible impedance mismatch between the reflecting layers of the stack so as to maximize bandwidth, and which reduces manufacturing costs by requiring fewer layers for the device, as compared to conventional acoustic mirrors.
The present invention provides a reflector stack for an acoustic device which comprises alternating layers of materials having a substantial acoustic impedance mismatch therebetween. In one aspect, the reflecting stack incorporates higher impedance layers of material that are determined based on the elastic constants and density of the materials, with known low impedance material layers to achieve a desired large mismatch.
In another aspect, the reflecting stack is formed from a low density material that is deposited by chemical vapor deposition (CVD) and/or sputtering on a substrate. The aforementioned higher impedance materials calculated as a function of their elastic constants may be combined with the low density material formed by CVD and/or sputtering to further increase the acoustic impedance mismatch, as well as to reduce the number of mirror layers required in the reflecting stack of the acoustic device.